The present invention relates to a pressure sensitive adhesive sheet employed for protecting circuit surfaces at the time of wafer back grinding. More particularly, the present invention relates to a pressure sensitive adhesive sheet employed in a wafer working process in which a wafer back is ground to thereby reduce the thickness of the wafer and finally effect the division thereof into individual chips.
In recent years, the spread of IC cards has been promoted, and further reduction of the thickness thereof is now demanded. Accordingly, it is required that the thickness of semiconductor chips, which has been about 350 xcexcm, be reduced to 50-100 xcexcm or less.
It is common practice to grind a wafer back after the formation of circuit patterns. When it is intended to grind a wafer to a thickness of, for example, 100 xcexcm or less, the strength of the wafer is decreased with the result that the wafer tends to be broken. In particular, the wafer is brittle, so that, when there is even a slight defect in the wafer, it may occur that cracking is caused by the defect and the crack grows large through the wafer to thereby extremely lower the chip yield.
Japanese Patent Laid-open Publication No. 5(1993)-335411 discloses a process for producing semiconductor chips, in which, for preventing the breakage of wafer attributed to the above growth of cracks, grooves of given death are formed from the wafer surface and thereafter the wafer back is ground.
In this chip producing process, the growth of cracks in the wafer from defects as starting points can be stopped by the grooves.
However, chip breakage has occurred in this method as well. Just before the division of the wafer into individual chips, the chips are in the state of being tied with each other by means of extremely thin bridges. Referring to FIG. 6, the chip brought into contact with a spindle of a back grinder and being currently ground differs from the neighboring chip not brought into contact with the spindle regarding the direction of application of grinding force and the magnitude of applied grinding force. This force difference causes chip dislocation and cracks the above thin bridges to thereby result in chip breakage.
Especially, in the use of conventional widely employed surface protective sheets, the degree of chip dislocation is large and chip breakage is likely to occur because the pressure sensitive adhesive of the surface protective sheets is relatively soft.
The present invention overcomes the problems heretofore encountered in the prior art. The present invention provides surface protective sheet which is applicable to a process enabling production of extremely thin IC chips with a high yield.
In one aspect of the present invention, a surface protective sheet for semiconductor wafer is provided which is used in wafer back grinding during a process comprising forming grooves on the surface of semiconductor wafer furnished with a circuit so that the cutting depth of the grooves is smaller than the thickness of the wafer, and grinding the back of the wafer so that the thickness of the wafer is reduced and that the wafer is finally divided into individual chips.
This surface protective sheet is comprised of a substrate and, superimposed thereon, a pressure sensitive adhesive layer having an elastic modulus of at least 1.0xc3x97105 Pa at 40xc2x0 C.
It is preferred that the pressure sensitive adhesive layer is composed of an energy radiation curable pressure sensitive adhesive. Further, it is preferred that the pressure sensitive adhesive layer have a shear peel strength of at least 10 kg/cm2.
Still further, in the present invention, it is preferred that the surface protective sheet have a compressive strain of 0.1 to 5.0 % at 40 kg/cm2.
The use of the surface protective sheet of the present invention enables the production of extremely thin IC chips with high yield.
In another aspect of the present invention, there is provided a method of grinding a wafer back, comprising the steps of:
forming grooves on the surface of semiconductor wafer furnished with a circuit so that the cutting depth of the grooves is smaller than the thickness of the wafer,
sticking the above surface protective sheet to the wafer surface furnished with semiconductor circuits, and
grinding the back of the wafer so that the thickness of the wafer is reduced and that the wafer is finally divided into individual chips.